The bulk micromachining process is a common scheme adopted in the micro-electro-mechanical system (MEMS) application, whereby a specific and desired structure is achievable on a silicon substrate or a silicon-on-insulator (SOI) substrate by means of an etching procedure in the bulk micromachining process.
Regarding the so-called surface acoustic wave (SAW) device, it is now popularly applied in electric equipments such as the television and the video recorder. Particularly, the SAW device plays an important role in the high frequency wireless and handset communication application due to the characteristics of miniature volume and low power loss.
Various bulk substrates of piezoelectric materials including LiNbO3 and quartz are applicable to the SAW device. Nowadays, many efforts are done not only for achieving the decrement of fabrication cost for the SAW device and the improvement of applicable frequency therefore, but also for integrating the fabrication for the SAW device with the conventional integrated circuit (IC) production. As a result, it is a great tendency toward fabricating the thin film SAW device that provides a high wave velocity by means of a procedure integrated with the standard silicon process technology. In such an integrated procedure for fabricating the SAW device, the piezoelectric thin films such as aluminum nitrides, zinc oxides and lead zirconate titanates (PZT) are utilized. The mentioned piezoelectric thin films are advantageous in their respective excellent piezoelectric property, high electromechanical coupling coefficient and high surface acoustic wave velocity. In addition, such piezoelectric thin films are capable of being compatible with the existing process for the semiconductor. Regarding the fabrication for the high frequency SAW device, the difficult and complicated procedure of a sub-micron process is not necessary anymore if the mentioned piezoelectric thin films are applied, which is regarded as a benefit resulting from the high surface acoustic wave velocity owned by the mentioned piezoelectric thin films.
However, such a thin film surface acoustic wave device is disadvantageous in the velocity dispersion. It is known that the wave velocity of the device would be significantly affected by the thickness of the substrate. That is, the surface wave would be inversely influenced by the substrate since the piezoelectric thin film is much thinner with respect thereto, so that the insertion loss would be decreased.
In order to overcome the above drawbacks in the prior art, the present invention provides a novel surface acoustic wave device and a novel fabrication method therefore. The provided surface acoustic wave device is designed as a suspended structure so as to decrease the thickness of the substrate, and thereby the velocity dispersion effect of the surface acoustic wave device is reduced. Moreover, the decrement of the insertion loss is also achievable through the surface acoustic wave device of the present invention. Therefore, the provided surface acoustic wave device has a great potential for being popularized.